Abstract
Nowadays, uninterruptible power supplies (UPS) play an important role in feeding critical loads in the electric power systems such as data centers or large communication hubs. Due to the increasing power of these loads and frequent need for expansion or redundancy, UPS systems are frequently connected in parallel. However, when UPS systems are parallel-connected, two fundamental requirements must be verified: potential circulating currents between the systems must be eliminated and the load power must be distributed between the systems according to UPS systems availability. Moreover, a high-quality load voltage waveform must be permanently ensured. In this paper innovative control strategies are proposed for paralleled UPS systems based on Finite Control Set Model Predictive Control (FCS-MPC). The proposed strategies simultaneously provide: controlled load power distribution, circulating current suppression and a high-quality load voltage waveform. A new dynamic converters deactivation mechanism is proposed. This new technique provides improved overall system efficiency and reduced power switches stress. In this paper, two multilevel based UPS systems are parallel-connected. Each UPS contains two three-level Neutral Point-Clamped-Converters (3LNPC) and a three-level DC-DC converter. The presented experimental results demonstrate the effectiveness of the proposed control strategies in several operating conditions.
Highlights
Uninterruptible Power Supplies (UPS) are used to feed a wide range of electrical loads: from low-power applications to high-power applications of several Megawatt
For the prediction of the system state at k + 1, each UPS system considers its own impact on the zero sequence circulating current (ZSCC), and the impact of the state applied at k by the other UPS, in the form of the common mode voltage generated by the load-side converter (LSC) and grid-side converter (GSC)
To choose the control sampling time, several experiments were made in order to select the minimum possible value, with which the control algorithm of both UPS systems can be executed without processor overrun occurring
Summary
Uninterruptible Power Supplies (UPS) are used to feed a wide range of electrical loads: from low-power applications (such as domestic computers and networks) to high-power applications of several Megawatt (such as data centers and medical facilities). In this study the load voltage references are generated based on a droop control scheme, which for some types of loads may represent a tradeoff between a precise load-sharing and a high-quality load voltage waveform Given all these facts, a cooperative-based FCS-MPC strategy that considers the cooperation potential between the converters within a UPS and between the converters of both UPS systems, is proposed in this paper to control all converters of two complete paralleled UPS systems based on a multilevel topology. The experimental results demonstrate that even for asymmetric paralleled systems, the proposed control strategies simultaneously ensure at any operating condition: a perfectly controlled load power distribution, circulating current suppression and a high-quality load voltage waveform.
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